Method for manufacturing fine tungsten powder

ABSTRACT

A method for finely powdering tungsten powder, which includes electrolytically oxidizing tungsten powder while stirring in an aqueous mineral-acid solution to form an oxide film in the surface of the tungsten powder and removing the oxide film with an alkaline aqueous solution; a method for producing tungsten powder to obtain fine tungsten powder by a process including the above method for finely powdering; and a tungsten powder having an average particle size of 0.04 to 0.4 μm, in which the dMS value (product of an average particle size d (μm), true density M (g/cm 3 ) and BET specific surface area S (m 2 /g)) is within the range of 6±0.4.

TECHNICAL FIELD

The present invention relates to a method for producing fine tungstenpowder. Specifically, the present invention relates to a method forprocessing a tungsten powder into the one having a smaller particle sizewhich is useful for use in an electrolytic capacitor; and a method forproducing fine tungsten powder using the above method.

BACKGROUND ART

With the progress of small-size, high-speed and lightweight electronicdevices such as cellular phones and personal computers, the capacitorused for these electronic devices is demanded to have a smaller size, alarger capacitance and a lower ESR (Equivalent Series Resistance).

As an example of such a capacitor, the electrolytic capacitor has beenproposed, which capacitor is produced by anodically oxidizing an anodebody for capacitors comprising a sintered body made of a valve-actingmetal powder which can be anodized such as tantalum to form a dielectriclayer made of the oxide of the metal in the surface of the anode body.

The electrolytic capacitor using tungsten as a valve-acting metal andemploying the sintered body of the tungsten powder as an anode body canattain a larger capacitance compared to the electrolytic capacitorobtained with the same formation voltage by employing the anode body ofthe same volume using the tantalum powder having the same particle size.However, the electrolytic capacitor having the sintered body of thetungsten powder has been unpracticed as an electrolytic capacitor due tothe large leakage current (LC). In order to solve this issue, acapacitor using the alloy of tungsten and other metals has been studiedand has achieved some improvement in the leakage current, but it was notenough (JP-A-2004-349658 (U.S. Pat. No. 6,876,083 B2); Patent Document1).

Patent Document 2 (JP-A-2003-272959) discloses a capacitor using anelectrode of a tungsten foil having formed thereon a dielectric layerselected from WO₃, W₂N and WN₂, but the capacitor is not to solve theabove-mentioned leakage current problem.

Also, Patent Document 3 (WO 2004/055843 publication (U.S. Pat. No.7,154,743 B2)) discloses an electrolytic capacitor using an anode bodyselected from tantalum, niobium, titanium and tungsten, but it does notdescribe a specific example using tungsten in the specification.

In an anode body for an electrolytic capacitor which is obtained bymolding tungsten powder and sintering it, the smaller particle sizeenables the production of an anode body having a larger capacitance, ifthe volume of the anode body is the same. Therefore the smaller size ofthe raw material tungsten powder is more preferable but the averageparticle size of a commercially-available tungsten powder is 0.5 to 20μm.

Tungsten powder can be manufactured by treating oxide, halide orammonium salt of tungsten as a raw material with a reducing agent suchas hydrogen. However, increase in the rate of reduction may give rise toa problem of generating a composite oxide and the like.

Therefore, it is necessary to decrease the rate of reduction in order toproduce finer powder, and it leads to low the production efficiency andhigh cost. Also, it is necessary to produce the fine powder by acomplicated process equipped with an expensive controlling device.Furthermore, there has been a problem of handling a material having awide explosibility range such as hydrogen gas.

PRIOR ART Patent Document

Patent Document 1: JP-A-2004-349658

Patent Document 2: JP-A-2003-272959

Patent Document 3: WO 2004/055843

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide a method for processingtungsten powder to obtain tungsten powder having a smaller particle sizeas a material of a capacitor comprising tungsten as an anode(hereinafter referred to as a tungsten capacitor), and a method forproducing fine tungsten powder using the method.

Means to Solve the Problem

As a result of intensive study to solve the above-mentioned problem, thepresent inventors have found that a fine tungsten powder which is moresuitable for a capacitor can be obtained by electrolytic oxidization ofthe surface of a tungsten powder which is currently available and haveaccomplished the present invention.

That is, the present invention relates to a method for finely powderingtungsten powder and a method for producing tungsten powder as below.

[1] A method for finely powdering tungsten powder, comprisingelectrolytically oxidizing tungsten powder while stirring in anelectrolytic solution to form an oxide film in the particle surface ofthe tungsten powder and removing the oxide film with an alkaline aqueoussolution.

[2] The method for finely powdering tungsten powder as described in [1]above, wherein the removal of the oxide film with an alkaline aqueoussolution includes mechanically removing the reaction product in theparticle surface of the tungsten powder.

[3] The method for finely powdering tungsten powder as described in [1]or [2] above, wherein the electrolytic solution is an aqueous solutionof mineral acid.

[4] The method for finely powdering tungsten powder as described in [3]above, wherein the mineral acid is selected from phosphoric acid, nitricacid, hydrochloric acid, boric acid and sulfuric acid.

[5] The method for finely powdering tungsten powder as described in [4]above, wherein the mineral acid is phosphoric acid or boric acid.

[6] A method for finely powdering tungsten powder, comprising dispersingtungsten powder in an aqueous solution containing an oxidizing agent toform an oxide film in the surface of the tungsten powder, removing theoxide film with an alkaline aqueous solution, followed by the methoddescribed in any one of [1] to [5] above.

[7] A method for producing fine tungsten powder, comprising obtainingtungsten powder having an average particle size of 0.04 to 0.4 μm by aprocess including the method described in any one of [1] to [6] above.

[8] A method for producing fine tungsten powder, comprising obtainingtungsten powder, in which the product of an average particle size (μm),true density (g/cm³) and BET specific surface area (m²/g) is within therange of 6±0.4, by a process including the method described in any oneof [1] to [6] above.

[9] A tungsten powder having an average particle size of 0.04 to 0.4 μm,in which the dMS value (product of an average particle size d (μm), truedensity M (g/cm³) and BET specific surface area S (m²/g)) is within therange of 6±0.4.

EFFECTS OF THE INVENTION

According to the present invention, using a currently available tungstenpowder or a tungsten powder which can be produced by a known method, atungsten powder having a small particle size and a substantiallyspherical shape which is suitable for an electrolytic capacitor can beobtained.

Since the tungsten powder obtained by the present invention have a smallparticle size, a capacitor obtained thereof has a large capacitance.Also, the tungsten powder has a high flowability due to the morespherical particle shape. Accordingly, the powder can be handled moreeasily in the process of producing granulate powder and the like.

MODE FOR CARRYING OUT THE INVENTION [Raw Material Tungsten Powder]

The average particle size of the raw material tungsten powder to befinely powdered in the present invention is preferably within the rangeof 0.1 to 10 μm.

A raw material tungsten powder can be obtained by , in addition to usinga commercially-available product, manufacturing by a known method. Forexample, it can be obtained by manufacturing by appropriately selectingfrom a method of crushing tungsten trioxide powder under hydrogenatmosphere; a method of reducing tungsten acid or tungsten halide withhydrogen or sodium and the like. Also, a tungsten powder may be obtainedby reducing the tungsten-containing mineral directly or through severalsteps and by selecting reducing conditions.

However, since it is difficult to obtain a raw material tungsten powderhaving a small particle size by these methods, a tungsten power treatedby chemical oxidization in advance as mentioned below or a fine particletungsten powder obtained according to the method of the presentinvention may be used as a raw material tungsten powder. Using thesetungsten powers having been finely-powdered as a raw material, atungsten powder having an even smaller particle size can be obtained.Thus, a tungsten powder having an average particle size of, for example,0.04 μm or less can be obtained by repeatedly applying the method of thepresent invention.

However, in the case of forming a dielectric layer by anode oxidation,there is a lower limit of the particle size of the powder which can besuitably used for a capacitor. The lower limit of the particle size ofthe tungsten powder used for a capacitor is twice the thickness of thedielectric layer to be formed. For example, when the rated voltage is1.6 V, the lower limit of the particle size is to be 0.04 μm. If theparticle size is smaller than the lower limit, a conductive portion ofthe tungsten is not left sufficiently when performing anodic oxidationand it becomes difficult to construct an anode of an electrolyticcapacitor.

Specifically, when used for a high capacitance capacitor having lowrated voltage, the particle size of the tungsten powder is preferably0.04 to 0.4 μm, more preferably 0.08 to 0.2 μm.

The raw material tungsten powder used in the method of the presentinvention may contain impurities within a range which does not affectthe capacitor properties or may be processed to contain elements such assilicon, nitrogen, carbon, boron, phosphorus and oxygen in order toimprove the capacitor properties. However, it is preferable that theparticle surface treatment such as silicidation, nitridation,carbonization or boronization to be described later is conducted in aprocess later than applying the present invention.

In the present invention, a finely powdered tungsten powder is obtainedby oxidizing the surface of particles of a raw material tungsten powderand removing the oxide film in the surface.

Oxidization of the particle surface of a tungsten powder can beperformed either by chemical oxidization or electrolytic oxidization,and a method of fine powdering using electrolytic oxidization (referredto as electrolytic oxidization method hereinafter) can be operated moreeasily than a method of fine powdering using chemical oxidization,because generation amount of the oxide film can be controlled only byadjusting an applied voltage during the electrolytic oxidization.Accordingly, the electorlytic oxidization method can be preferablyapplied to production of a finer tungsten powder in which generationamount of the oxide film needs to be controlled more precisely.

Fine powdering of tungsten powder may be performed only by theelectrolytic oxidization method. In the case of fine powdering ofrelatively large particles (for example, average particle size of 1 μmor more), tungsten powder may be pre-treated by chemical oxidization forfine powdering to some extent (for example, to average particle size of0.5 μm or less) before applying the electrolytic oxidization method, andthe applied voltage at the time of electrolytic oxidization can bereduced, which makes the operation easier.

(1) Electrolytic Oxidization Method

Oxidization of surface of tungsten powder particle:

As an electrolyte, an electrolyte solution such as an aqueous solutionof mineral acid and salt thereof can be used, and aquueous solution ofmineral acid is preferable because washing after oxidization is easy.Examples of mineral acid include phosphoric acid, nitric acid,hydrochloric acid, sulfuric acid and boric acid, and phosphoric acid orboric acid are preferable from the viewpoint that an oxide film havingdefects is obtained relatively easily, and the oxide film is easy to beremoved with alkali aqueous solution later. Preferable concentration ofthe mineral acid aqueous solution is 0.1 to 5 mass %. If theconcentration increases, cleaning of tungsten powder in the subsequentstep becomes complicated.

The electrolytic oxidization is, for example, conducted as below. Amaterial tungsten powder is put in a metal container containing anelectrolyte while stirring, a predetermined voltage is applied between ametal stirring stick as an anode and the container as a cathode, and thetungsten powder is oxidized by applying a current at a temperaturepreferably from room temperature to the boiling point of the aqueoussolution, more preferably 30° C. to 80° C. for preferably 10 minutes to100 hours and more preferably 1 to 10 hours. A solvent component issupplied in the amount corresponding to the amount lost by evaporation,as needed.

The applied voltage may be set depending on the desired degree of finepowdering. Higher applied voltage leads to an increased amount of oxidefilm which results in smaller particle size. Specific voltage can bedetermined by a preliminary experiment. However, it takes time forelectrolytic oxidization under high voltage, and preferable appliedvoltage may be set at 100V or less, and more preferably 50V or less, andthe operation of fine powdering may be repeated, if necessary.

After the electrolytic oxidization is finished, the operation ofremoving the liquid by decantation and the like is repeated, and thetungsten powder is washed with a solvent such as water. The color of thetungsten in this state changes from black to yellowish blue.

Removal of the oxide film:

The oxide film of the tungsten powder obtained as mentioned above, inwhich the surface is oxidized, is subjected to treatment with analkaline aqueous solution and removed at least chemically.

Preferably, using a device such as a homogenizer which is capable ofvigorous stirring, the above-mentioned stirring is conducted whilemechanically removing the product generated in the surface of tungstenparticles as well.

As an alkaline solution, for example, sodium hydroxide aqueous solution,potassium hydroxide aqueous solution, ammonia water and the like can beused, and sodium hydroxide aqueous solution and potassium hydroxideaqueous solution are preferable.

Specifically, an alkaline aqueous solution is added to a tungsten powderin which the surface is oxidized. The solution is allowed to stand afterstirring. After removing the solution by decantation, a series ofoperations of feeding a solvent such as water into the tungsten powder,stirring the resultant solution, allowing it to stand and subjecting itto decantation is repeated several times. By these operations, the colorof the tungsten powder becomes black, and the oxide formed in thesurface of tungsten particles is removed. Subsequently, the solution isdried in a vacuum dryer under reduced pressure (e.g. with the reducedpressure reduced of 10⁴ to 10² Pa at the temperature of 50 to 180° C.)and cooled to room temperature. Then, by gradually introducing air intothe dryer so that ignition may not occur and taking out the powder intothe air, a tungsten powder having a smaller particle size than that ofthe raw material tungsten powder can be obtained.

(2) Chemical Oxidization Method

In the chemical oxidization carried out as a pre-treatment, if desired,a raw tungsten powder is dispersed in an aqueous solution of oxidant bystirring and the like and retained for a predetermined time to oxidizethe surface of the tungsten powder. A device such as a homogenizer whichis capable of vigorous stirring is preferably used in order to keep agood dispersion state and make the surface of the tungsten powderoxidized rapidly. Further, oxidization proceeds at a faster pace at ahigh temperature.

Examples of the oxidant include a manganese (VII) compound such aspermanganate; a chrome (VI) compound such as chromium trioxide, chromateand dichromate; a halogen acid compound such as perchloric acid,chlorous acid, hypochlorous acid and salt thereof; peroxide such ashydrogen peroxide, diethyl peroxide, sodium peroxide and lithiumperoxide; peroxoacid such as peracetic acid and persulfate and saltthereof. Particularly preferable are hydrogen peroxide and ammoniumpersulfate, due to its handleability, stability as an oxidant and highsolubility to water.

Concentration of the oxidant in an aqueous solution is within a range ofabout 1% to a saturated solubility of the oxidant. Concentration of theoxidant can be appropriately determined by a preliminary experiment.

The time period for oxidization is one hour to 1,000 hours andpreferably one hour to 100 hours. Oxidization temperature is from roomtemperature to the boiling point of the solvent and preferably 50° C. tothe boiling point of the solution.

After oxidization reaction, tungsten powder is taken out from theoxidization reaction solution by decantation and the like, and a seriesof operations of feeding the tungsten powder into a solvent, stirringthe resultant solution, allowing it to stand and subjecting it todecantation is repeated for washing the tungsten powder. The color ofthe tungsten at this point changes from black of the raw material toyellowish blue, and it can be visually confirmed that the surface of thetungsten powder is oxidized.

Regarding a solvent used in each step of the present invention, not onlywater but also a mixed aqueous solution of water and water-solubleorganic solvent (e.g. ethanol and methanol) can be selected from theviewpoint of dispersibility of the powder and time required fordecatation.

Removal of oxide film of the tungsten powder in which the surface isoxidized is conducted in a similar manner to the removal of the oxidefilm in the electrolytic oxidization method.

According to the method of the present invention, almost sphericaltungsten particles can be obtained unless the raw material tungstenpowder particles have a shape having especially high anisotropy. Thefact that the particle shape is spherical can be confirmed by that theaverage particle diameter (d) (μm), true density (M) (g/cm³) and BETspecific surface area (S) (m²/g) of the obtained tungsten powder satisfythe following formula.

d=6/(M×S)   (1)

That is, it can be said that if the product (d×S×M; abbreviated as dSM)of the average particle diameter (d) (μm), true density (M) (g/cm³) andBET specific surface area (S) (m²/g) of the obtained tungsten powder isnear to 6, the obtained tungsten powder particles have aalmost-spherical shape. The dMS value of the tungsten powder obtained bythe present invention is generally within the range of 6±0.4.Furthermore, by using the tungsten powder obtained by applying themethod of the present invention as a raw material powder, it is alsopossible to obtain a tungsten powder composed of particles with higherdegree of sphericity.

Since the dielectric layer formed in the surface of an almost-sphereparticle has a substantially constant curvature and has no highly-curvedportion with a small curvature in which stress tends to be concentrated,it undergoes little degradation. As a result, a capacitor having betterLC characteristics can be obtained.

The tungsten powder produced by the method of the present invention maybe directly sintered to be made into a sintered body, or may begranulated into granules about the size of 10 to 300 μm to be sinteredand made into a sintered body. The granulated tungsten powder is easierto handle and to keep the ESR as low as possible.

Furthermore, the tungsten powder produced by the method of the presentinvention may be subjected to silicidation, nitridation, carbonizationor boronization treatment to be made into a tungsten powder containingat least one of tungsten silicide, tungsten nitride, tungsten carbideand tungsten boride in a part of the surface of the tungsten particles.These treatments may be conducted when the tungsten powder has become agranulated powder or a sintered body. An electrolytic capacitor isfabricated comprising the sintered body as one electrode (anode), acounter electrode (cathode) and a dielectric body interposedtherebetween.

EXAMPLES

The present invention is described below by referring to Examples andComparative Examples, but the present invention is not limited thereto.

In the present invention, the particle diameter, specific surface areaand true density were measured by the methods described below.

The particle diameter was measured by using HRA9320-X100 manufactured byMicrotrack Inc. and the particle size distribution was measured by thelaser diffraction scattering method. A particle diameter value (D₅₀; μm)corresponding to cumulative volume % of 50 volume % was designated asthe average particle size (d). The diameter of the secondary particlesis to be measured by this method. However, since the tungsten powdergenerally has good dispersibility, the measurement results near to theprimary powder particle diameter can be obtained. Therefore, themeasurement results can be substantially regarded as a primary particlediameter and applied to the above-described formula (1) to judge theparticle shape.

The specific surface area (S; m²/g) was measured by the BET method byusing NOVA2000E (manufactured by SYSMEX Corp.)

The true density (M; g/cm³) was measured by a picnometer method (20°C.).

Example 1

200 g of tungsten powder having an average particle diameter of 1 μmobtained by reducing ammonium tungstate with hydrogen was put in 500 mlof distilled water in which 5 mass % of ammonium persulfate wasdissolved and stirred at 50° C. for 24 hours using homogenizer NS-51manufactured by MICROTEC Co., Ltd. An amount of water lost throughevaporation was successively added all the period. After allowing theliquid to stand at room temperature for 17 hours to sedimentate thepowder, the liquid was removed by decantation. After adding another 200ml of distilled water, the liquid was stirred by a homogenizer for fiveminutes and allowed to stand for several hours, and the liquid wasremoved by decantation. The series of operations of feeding distilledwater, stirring, allowing the water to stand, and decantation wasrepeated four times. The tungsten powder at this point underwent achange in color to yellowish blue, which revealed that the surface ofthe tungsten powder was oxidized. Subsequently, 100 ml of a 5 mass %sodium hydroxide aqueous solution was added to the powder and stirred bya homogenizer for one hour. As described above, after allowing theliquid to stand and removing the liquid by decantation, the series ofoperations of feeding distilled water, stirring, allowing the liquid tostand, and decantation was repeated four times. The tungsten powder atthis point was black and the oxide formed in the particle surface wasremoved. The produced powder had an average particle diameter of 0.5 μm.

Next, a tungsten powder containing water after decantation (100 g forthe powder itself) was moved to a separately prepared container made ofstainless steel, and 300 ml of 1 mass % phosphoric acid aqueous solutionwas fed as an electrolytic solution. A stirring stick made of stainlesssteel (four 4 cm-long blades made of stainless steel are set in thelower part of the stick, at an angle of 90° to each other) is placed inthe electrolytic solution from the upper part of the container, andelectrolytic oxidization was conducted by applying a voltage of 20 Vbetween the stirring stick as an anode and the container as a cathode at50° C. for five hours while stirring the solution at a rotation speed of100 rpm. An amount of water lost through evaporation was sucessivelyadded all the period. After allowing the liquid to stand at roomtemperature for 40 hours to sedimentate the powder, the liquid wasremoved by decantation. After adding another 200 ml of distilled water,the liquid was stirred by the stirring stick for 20 minutes and allowedto stand for 20 hours, and the liquid was removed by decantation. Theseries of operations of feeding distilled water, stirring, allowing thewater to stand, and decantation was repeated four times. The tungstenpowder at this point underwent a change in color to yellowish blue,which revealed that the surface of the tungsten powder was oxidized.

Subsequently, 100 ml of a 5 mass % sodium hydroxide aqueous solution wasadded to the powder and stirred by the stirring stick for one hour. Asdescribed above, after allowing the liquid to stand and removing theliquid by decantation, the series of operations of feeding distilledwater, stirring, allowing the liquid to stand, and decantation wasrepeated four times. The tungsten powder at this point was black and theoxide formed on the particle surface was removed. Subsequently, a partof the tungsten powders is moved to a vacuum dryer and dried underreduced pressure at the temperature of 50° C., and cooled to roomtemperature. Then, air was introduced gradually into the dryer so thatignition may not occur, and the powder was taken out into the air. Theproduced powder had an average particle diameter (d) of 0.2 μm, specificsurface area (S) of 1.5 m²/g and true density (M) of 19.3. The productof the average particle diameter, specific surface area and true densityof the obtained tungsten powder (dMS) was 5.8.

Example 2

The electrolytic solution used in Example 1 was changed from 300 ml of 1mass % phosphoric acid aqueous solution to a mixed solution of 100 ml of1.5 mass % boric acid methanol and 350 ml of water. Decantation liquidwas changed to a mixed solution of methanol and water in the sameproportion as above. Also, an amount of methanol lost throughevaporation was successively added all the period. A tungsten powder wasobtained in a similar manner as in Example 1 except the above. Theproduced powder had an average particle diameter (d) of 0.16 μm,specific surface area (S) of 2.0 m²/g and true density (M) of 19.3. Theproduct of the average particle diameter, specific surface area and truedensity of the obtained tungsten powder (dMS) was 6.2. It was confirmedthat the particles of the powder obtained both in Examples 1 and 2 werealmost spherical because the dMS value was within the range of 6±0.2.

1. A method for finely powdering tungsten powder, comprisingelectrolytically oxidizing tungsten powder while stirring in anelectrolytic solution to form an oxide film in the particle surface ofthe tungsten powder and removing the oxide film with an alkaline aqueoussolution.
 2. The method for finely powdering tungsten powder as claimedin claim 1, wherein the removal of the oxide film with an alkalineaqueous solution includes mechanically removing the reaction product inthe particle surface of the tungsten powder.
 3. The method for finelypowdering tungsten powder as claimed in claim 1, wherein theelectrolytic solution is an aqueous solution of mineral acid.
 4. Themethod for finely powdering tungsten powder as claimed in claim 3,wherein the mineral acid is selected from phosphoric acid, nitric acid,hydrochloric acid, boric acid and sulfuric acid.
 5. The method forfinely powdering tungsten powder as claimed in claim 4, wherein themineral acid is phosphoric acid or boric acid.
 6. A method for finelypowdering tungsten powder, comprising dispersing tungsten powder in anaqueous solution containing an oxidizing agent to form an oxide film inthe surface of the tungsten powder, removing the oxide film with analkaline aqueous solution, followed by the method claimed in claim
 1. 7.A method for producing fine tungsten powder, comprising obtainingtungsten powder having an average particle size of 0.04 to 0.4 μm by aprocess including the method claimed in claim
 1. 8. A method forproducing fine tungsten powder, comprising obtaining tungsten powder, inwhich the product of an average particle size (μm), true density (g/cm³)and BET specific surface area (m²/g) is within the range of 6±0.4, by aprocess including the method claimed in claim
 1. 9. A tungsten powderhaving an average particle size of 0.04 to 0.4 μm, in which the dMSvalue (product of an average particle size d (μm), true density M(g/cm³) and BET specific surface area S (m²/g)) is within the range of6±0.4.